This invention relates to a leakage current detecting structure which is intended to detect even small leakage currents.
FIG. 1 illustrates the operation of a conventional leakage current detector. As shown in FIG. 1, electronic equipment 1 is connected through lead wire L.sub.1 and L.sub.2 to a power source 2. The lead wires L.sub.1 and L.sub.2 are surrounded by an annular iron core 3, on which a coil 4 is wound. The coil 4 is connected to a current detecting circuit 5. Therefore, when no current I.sub.l leaks from the electronic equipment 1, a "forward" current I.sub.1 in the lead wire L.sub.1 is equal in value to a "return" current I.sub.2 in the lead wire L.sub.2, and therefore no magnetic flux is induced in the iron core 3. Specifically, the magnetic flux .phi. induced by the current I.sub.1 and the magnetic flux .phi. induced by the current I.sub.2 cancel each other. On the other hand, where a leakage current I.sub.l exists, the "return" current I.sub.2 is smaller by the leakage current I.sub.l than the current I.sub.1. Accordinglly, a magnetic flux .phi..sub.l corresponding to the difference between the current I.sub.1 and the current I.sub.2, namely, the leakage current I.sub.l, is induced in the iron core 3. As a result, an induction current I.sub.3 is induced in the coil 4 by the magnetic flux thus induced. The current detecting circuit 5 detects the induction current I.sub.3 to thereby detect the magnitude of the leakage current I.sub.l.
In the above-described technique, the difference between the current I.sub.1 and the current I.sub.2 is converted into magnetic flux, and the magnetic flux is then converted into an induction current I.sub.3, with an energy loss. Accordingly, this technique cannot detect a leakage current with high accuracy; it can detect only rather large leakage currents (at least several tens of milli-amperes (mA)). Therefore, there has been a strong demand for a means capable of detecting small leakage currets, especially in the case of medical equipment, in which a small leakage current on the order of several tens of microamperes (.mu.A) to several hundreds of micro-amperes (.mu.A) must be detected.